Re: focus

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On  8 May 99 at 13:43, Llamapix@xxxxxxx wrote:

> Hi all.  A question regarding focusing which is not addressed in anything 
> I've read about IR so far.  I am shooting Kodak HIE with a 25 red filter.    
> I use a Canon AT-1 with Canon 28mm and 50mm lenses.
> 
> I have been shooting portraits with IR.  At first I shot some semi low-light 
> pictures using larger apertures like f/3.5  In spite of careful focusing and 
> offsetting to the IR mark, it seemed the subjects' faces were slightly out of 
> focus.  I then theorized that perhaps since the film was being exposed by 
> both visible and infrared light, that by using the offset focus I was 
> therefore leaving all the visible light rays out of focus.  Is this the case?

Yes.

This one of the main problems of using IR-correction marks....the
correction needed not only depends on 1) sensitivity of film (the
Konica Hexar has two IR-correction settings, one for 750nm (Konica),
and one for 850nm (Kodak)), but also 2) on the filter used....the
stronger the filter, the smaller the spectral range you are
recording, and the easier it is to pin-point a certain correction
for it....without any filter, you would have to bridge a very large
range (350-950nm), hard to cover with a non-APO tele without
stopping down....but luckily HIE more or less requires a #25 red
filter already (which leaves only the smaller range 600-950nm).

Factor 3 in setting an IR-mark is the distance....normally the mark 
is set for infinity, which makes close-focus sometimes guess work, 
especially with complicated optical designs....zooms in particular.

All 3 factors combined are also the reason why Leica doesn't put 
IR-marks on their modern lenses anymore.... 
 
> Since then, I have been shooting with smaller apertures like f/8. and then 
> setting the focus in between the normal and IR focus marks.  The greater 
> depth of field easily encompasses both focus marks.  Yet in some of the 
> pictures people's faces still seem a little soft.  Is this just the nature of 
> IR?  Is it because of the high IR reflectance from people's skin combined 
> with halation perhaps that makes them appear soft?  What are other people's 
> experience with IR and portraits?

Skin comes across fuzzy indeed, that's part of HIE's
characteristics....keep in mind you can more or less look through
someones skin if he/she has pronounced veins (and a light skin
color)....;-)) 


--                 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
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